Lucky Star Escapes Black Hole With Minor Damage

Closest near-miss event to be spotted near the Milky Way

COLUMBUS, Ohio: Astronomers have gotten the closest look yet
at what happens when a black hole takes a bite out of a star—and the star lives
to tell the tale.

We may think of black holes as swallowing entire stars—or
any other object that wanders too close to their immense gravity. But sometimes,
a star that is almost captured by a black hole escapes with only a portion of
its mass torn off. Such was the case for a star some 650 million light years
away toward Ursa Major, the constellation that contains the “Big Dipper,” where
a supermassive black hole tore off a chunk of material from a star that got
away.

Astronomers at The Ohio State University couldn’t see the
star itself with their
All-Sky
Automated Survey for Supernovae
(ASAS-SN, pronounced “assassin”). But they
did see the light that flared as the black hole “ate” the material that it
managed to capture.

If Laniakea is our galactic “city,” this event—called a “
tidal
disruption event
,” or TDE— happened in our larger metropolitan area. Still,
it’s the closest TDE ever spotted, and it gives astronomers the best chance yet
of learning more about how supermassive black holes form and grow.

Krzysztof Stanek

Thomas Holoien

Christopher Kochanek

ASAS-SN has so far spotted more than 60 bright and nearby supernovae; one of the program’s other goals is to try to determine how often TDEs happen in the nearby universe. But study co-author Krzysztof Stanek, professor of astronomy at Ohio State, and his collaborators were surprised to find one in January 2014, just a few months after ASAS-SN’s four telescopes in Hawaii began gathering data.

To Stanek, the fact that the survey made such a rare find so quickly suggests that TDEs may be more common than astronomers realized.

“We found one right out of the gate,” he said. “Based on
that, we are encouraged that the rate may be higher than one TDE every year or
two.

“You could say we just got lucky, but when you get lucky
time after time, you’re doing something right,” he continued. “Maybe the rate
truly is higher than people expected, which would mean that we should be seeing
more of these in the near future.”

Doctoral student
Thomas Holoien led
the observations and analysis of the TDE when it first flared to brightness on
January 25, 2014. It appeared near the back left “foot” of Ursa Major, between
the stars Alula Borealis and Praecipua. He labeled the object ASASSN-14ae, and
at first he thought it was a supernova, albeit an unusual-looking one. But its brightness pattern ultimately indicated
something else, and he and his colleagues determined that they were seeing a
TDE.

Based on the amount of energy released during the event, the
researchers calculated that a relatively small amount of stellar material—only
one thousandth of the mass of our sun, an amount approximately equal to the
mass of the planet Jupiter—had been sucked into the black hole.

Study co-author
Christopher Kochanek,
professor of astronomy at Ohio State and the Ohio Eminent Scholar in
Observational Cosmology, was among the first experts to model TDEs 25 years ago,
as a graduate student. Only a handful have been spotted since then, however,
and nobody knows for sure how important a contribution TDEs might make to the growth
of black holes in the universe.

Conventional wisdom suggests that black holes don’t consume
whole stars all that often—maybe only once every 10,000-100,000 years, Kochanek
said. But how often black holes tear off just a piece of a passing star is an
open question.

“The issue is that the chances of a black hole partly
shredding a star may not be all that different from the chances of it
completely shredding a star. We just don’t know,” Kochanek said.